Hey Leo, let's clear up this confusion—you're not missing some hidden trick, just need to connect the dots between the type documentation and real-world code usage!

First off: What exactly is admin.auth.UserInfo? #

It's not a class you instantiate directly, but an interface/type definition that describes the properties a user-related object will have. Any user record you fetch or receive from the Firebase Admin SDK (like via getUser() or auth triggers) implements this interface, meaning it has all the fields documented under UserInfo (email, displayName, uid, etc.).

Here's how to actually use it in code #

You don't need to reference admin.auth.UserInfo directly in your JavaScript code. Instead, work with the user objects returned by the SDK's methods. Here's a concrete example:

const admin = require("firebase-admin");

// Don't forget to initialize the SDK first!
// (Use your service account key for production environments)
admin.initializeApp({
  credential: admin.credential.applicationDefault()
});

async function fetchUserDetails(uid) {
  try {
    // This returns a UserRecord object that implements UserInfo
    const user = await admin.auth().getUser(uid);
    
    // Now you can access all UserInfo properties directly from the user object
    console.log(`User Email: ${user.email}`);
    console.log(`Display Name: ${user.displayName || "No display name set"}`);
    console.log(`User UID: ${user.uid}`);
    console.log(`Profile Photo URL: ${user.photoURL || "No photo set"}`);
  } catch (error) {
    console.error("Failed to fetch user:", error.message);
  }
}

// Call the function with your target user's UID
fetchUserDetails("your-user-uid-here");

Another common use case: Auth triggers #

When you set up Cloud Functions triggers for auth events (like user creation), the event object gives you a user record that also uses the UserInfo interface:

exports.onUserSignUp = functions.auth.user().onCreate((user) => {
  // The 'user' parameter here implements UserInfo
  console.log(`New user signed up with email: ${user.email}`);
  console.log(`User provider: ${user.providerId}`);
  
  // You can even update the user's info using these properties
  return admin.auth().updateUser(user.uid, {
    displayName: `New User ${user.uid.slice(0, 4)}`
  });
});

Why the documentation might feel confusing #

If you're looking at the official docs, admin.auth.UserInfo is listed as a type reference—this is mostly for TypeScript developers who want type checking. In plain JavaScript, you don't need to import or reference this type; just use the properties directly from the user objects the SDK gives you.

内容的提问来源于stack exchange，提问作者Leo